together with the combined effect of the slightlysmaller errors on the age due to the greater spreadin Pb-Pb data to define the 207PbR/206PbR ratio,greater number of points defining the isochron,acceptable sample/blank ratios for all measure-ments used to define the array (table S4), and thelow amount of terrestrial Pb contamination (21).

Allende meteorite (Fig. 1), we used the well-constrained solar 238U/235U value of 137.786 T
0.013 to calculate the absolute Pb-Pb ages of our
subset of chondrules. In contrast to the narrow
age span defined by CAIs, the chondrule ages

Fig. 2. Pb-Pb isochron diagrams for Efremovka CAI 22E (A), Allende chon-drule C30 (B), and NWA 5697 chondrule C2 (C).Fig. 3. Initial Pb isotopic compositions of individualchondrules. The initial Pb isotope compositions are definedby the intersection of the individual isochrons and a Pbevolution array anchored on the solar system initial Pbisotope composition defined by the Nantan iron meteorite(14). The m values (238U/204Pb) of chondrules are indicatedin parentheses. Chondrule C30 was displaced to the right-hand side of the solar system initial Pb array for clarity. Theuncertainty of the solar system initial Pb value is smallerthan the symbol.